1. Field of the Invention
The present invention relates to a measuring apparatus, and more particularly to a measuring apparatus for measuring the thickness of a thin film formed on a surface of a workpiece such as a semiconductor substrate.
2. Description of the Related Art
In recent years, a higher integration of a semiconductor device has required the narrower wiring and the multilayer wiring, and hence it is necessary to make a surface of a semiconductor substrate highly planarized. Specifically, finer interconnections in highly integrated semiconductor devices have led to the use of light with shorter wavelengths in photolithography, so that a tolerable difference of elevation at the focal point on the substrate becomes smaller in the light with shorter wavelengths. Therefore, a difference of elevation at the focal point should be as small as possible, i.e., the surface of the semiconductor substrate is required to be highly planarized. One customary way of planarizing the surface of the semiconductor substrate is to remove irregularities (concaves and convexes) on the surface of the semiconductor substrate by a chemical mechanical polishing (CMP) process.
In the chemical mechanical polishing process, after a surface of a semiconductor substrate has been polished for a certain period of time, the polishing process should be finished at a desired position or timing. For example, some integrated circuit designs require an insulating film (layer) of SiO2 or the like to be left on a metallic interconnection of copper, aluminum, or the like. Since a metallic layer or other layers are further deposited on the insulating layer in the subsequent process, such an insulating layer is referred to as an interlayer. In this case, if the semiconductor substrate is excessively polished, the lower metallic layer is exposed on the polished surface. Therefore, the polishing process needs to be finished in such a state that a predetermined thickness of the interlayer remains unpolished.
According to another polishing process, interconnection grooves having a certain pattern are formed in a surface of a semiconductor substrate, and a cooper (Cu) layer is deposited on the semiconductor substrate to fill the interconnection grooves filled with copper or copper alloy, and then unnecessary portions of the Cu layer are removed by a chemical mechanical polishing (CMP) process. Specifically, the Cu layer on the semiconductor substrate is selectively removed by the chemical mechanical polishing process, leaving only the Cu layer in the interconnection grooves. More Specifically, the Cu layer is required to be removed until an insulating layer of SiO2 or the like is exposed in surfaces other than the interconnection grooves.
In such cases, if the semiconductor substrate is excessively polished until the Cu layer in the interconnection grooves is removed together with the insulating layer, then the resistance of the circuits on the semiconductor substrate would be so increased that the semiconductor substrate might possibly need to be discarded, resulting in a large loss of resources. Conversely, if the semiconductor substrate is insufficiently polished to leave the copper layer on the insulating layer, then interconnections on the semiconductor substrate would not be separated from each other as desired, but a short circuit would be caused between those interconnections. As a result, the semiconductor substrate would be required to be polished again, and hence its manufacturing cost would be increased. The above problems also occur when another metallic film of aluminum or the like is formed on a semiconductor substrate and polished by the CMP process.
Therefore, it has heretofore been proposed to detect an end point of the CMP process with use of a measuring apparatus having an electric current meter, an eddy current sensor, an optical sensor, or the like for measuring the thickness of an insulating film or a metal film formed on a polished surface to detect when the CMP process is to be finished. In a deposition process such as a plating process or a chemical vapor deposition (CVD) process, it has also been proposed to measure the thickness of a thin film deposited on a substrate to detect an end point of the process, as with the CMP process.
As semiconductor devices have been more highly integrated, the measuring apparatus has been required to measure the film thickness with higher accuracy. The need for such a highly accurate measuring apparatus has been increased not only in the field of semiconductor fabrication, but also in other industrial fields.